This invention relates to a carrier tape, and more particularly to a carrier tape of a COF (Chip On Film) type and its manufacturing method.
Carrier tapes, in which a plurality of flexible wiring substrates are continuously formed along their length, include those of a so-called TAB (Tape Automated Bonding) type and of a so-called COF (Chip On Film) type. The TAB type carrier tape includes a base film having through holes formed in portions near portions on which integrated circuits are formed, and formed in portions near outer leads. The COF type carrier tape includes a base film with no through holes formed in portions near portions on which integrated circuits are formed. These carrier tapes are widely used for the connection of, for example, drivers for use in liquid crystal display devices.
In the TAB type carrier tape, the base film is thick, and is bent at portions on which outer leads are provided, thereby connecting the outer leads to the terminals of an external circuit. To enhance the flexibility of the tape, through holes are formed in those portions of the base film which are located under the outer leads. On the other hand, in the COF type carrier tape, the base film, which is made of polyimide, is relatively thin and highly flexible. Therefore, no through holes are formed in portions of the tape near portions on which integrated circuits are formed.
As is shown in FIG. 35, a carrier tape 21 of the COF type has a plurality of corresponding sprocket holes formed along both left and right side portions of a base film 22, wirings 24 provided on middle surface portions of the base film 22, and protective films 25 each covering corresponding wirings 24 except for opposite terminal portions thereof. The base film 22 is as long as between several tens m to 100 m. After forming the sprocket holes, the base film is subjected to a roll-to-roll treatment using pin rollers.
Since the TAB type base film is generally formed of a polyimide film with a relatively thick thickness of about 75 to 125 .mu.m, the base film can have a sufficient strength even at portions near the sprocket holes. Accordingly, the pins of the pin roller are reliably engaged with the sprocket holes, thereby enabling smooth transfer of the carrier tape by the pin roller without deforming the sprocket holes.
However, there is a case where a relatively thin film with a thickness of 38 .mu.m or less is used as the COF type base film 22. In this case, the base film 22 does not have sufficient strength at portions near the sprocket holes 23. Therefore, it is difficult to suitably engage the pins of the pin roller with the sprocket holes 23. Even if the pins may be engaged with the holes, the carrier tape is liable to come away from the pins while it is transferred, and the four corners of each sprocket hole are liable to tear because of mechanical stress that concentrates on the portions of the carrier tape near the sprocket holes while the tape is transferred by the pins. The tears are liable to enlarge, which will cause displacement of the carrier tape 21 and hence will adversely affect the degree of alignment in a later process.